This application is based on and claims the benefit of European Patent Application No. 00402400.6, filed Aug. 31, 2000, which is incorporated by reference herein.
The invention relates to a process for producing a metal tube in accordance with the preamble of claim 1.
For the continuous production of metal tubes welded along their longitudinal seams, electrical seam welding processes, such as high-frequency pressure welding, resistance welding, arc welding, for instance TIG welding, or plasma welding have been known for a long time. These welding processes have proven successful for various materials and tube dimensions.
Most recently, laser welding has shown to be particularly suitable for certain materials and products, since laser welding offers some decisive advantages compared to conventional welding processes. For instance, the weld seam produced by laser welding is characterized by a slender seam geometry with a high depth/width ratio and a small heat affected zone. The reasons for these characteristics are the high, strictly limited energy input and the high welding rates.
While laser welding is already widely used for steel tubes, particularly tubes made of stainless steel or galvanized steel, or for aluminum, particularly copolymer-coated aluminum strip, this technique has not yet gained acceptance for copper or copper alloy tubes. This is due to the fact that copper, because of its bright surface, reflects the laser beam to such an extent that introducing the laser beam energy into the strip edges is prevented and welding of the strip edges is not possible.
Also, the high thermal conductivity of copper is a reason for the poor heat concentration in the area of the strip edges to be welded.
The object of the present invention is to define a process by means of which laser welded copper tubes with thin wall thicknesses can be produced in large lengths in a continuous process.
This object is attained by the combination of the characterizing features of claim 1.
Combining the features of roughening the surface of the copper tube in the area of its strip edges and wetting it with a liquid hydrocarbon makes it possible to substantially increase the introduction of the laser energy into the strip edge area. The explanation for the optimization by wetting with liquid hydrocarbon is thought to be that the hydrocarbon either deflects the laser beam, which is typically directed onto the strip edges at an angle of less than 90xc2x0 because of the refraction index of the hydrocarbon, or that the laser beam bums the hydrocarbon to produce a carbon film on the strip edges, which improves the introduction of the laser energy.